The present invention relates to a thermally-assisted magnetic recording head and a thermally-assisted magnetic recording apparatus, and more particularly, it relates to newly improved thermally-assisted magnetic recording head and thermally-assisted magnetic recording apparatus in which light irradiation is used to heat a magnetic recording medium and raise a temperature thereof to carry out magnetic recording, so that the very high density magnetic recording can be attained.
Magnetic recording apparatuses which magnetically record and reproduce information have been developed as large-capacity, high-speed, and inexpensive information storing means. Especially, recent advancement of hard disk drives (HDD) is remarkable to such an extent that recording density of finished commercial products reaches 10 Gb/in2 (Giga bytes per square inches) or over while an internal data transfer speed is 100 Mbps (Mega bytes per second) or above, and a price in mega byte is dropped to several yens/MB. Such advanced high-density HDDs have resulted from total advancement of a variety of technical factors including signal processing, servomechanisms, heads, mediums, HDI, and the like. For these years, however, thermal disturbance in mediums has been regarded as a cause of impeding further enhancement of the density of the HDDs.
Enhancement of the magnetic recording density is implemented by miniaturization of recording cells (recording bits). However, since signal magnetic field derived from the medium is diminished due to the miniaturization of the recording cells, it is essentially required reducing medium noise in order to ensure a specified signal to noise ratio (S/N). The medium noise is caused by disturbance in magnetization transit section, and a degree of the disturbance is proportional to a unit magnetic reversal. Although the magnetic medium made of thin film of polycrystalline magnetized particles (hereinafter referred to as “poly-particle thin film” or “poly-particle medium”) is used, the unit magnetic reversal in the poly-particle thin film consists of a numerous number of magnetized particles coupled to one another in a manner of exchange bonding when magnetic exchange interactions mutually affect those particles.
Under the conditions of the recording density ranging from several hundreds Mb/in2 to several Gb/in2, in the prior art, noise reduction in the medium has been implemented by diminishing the exchange interactions between magnetized particles and decreasing the unit magnetic reversal. As to recent magnetic mediums of the recording density of 10 Gb/in2, the unit magnetic reversal is compressed down to two to three magnetized particles, and in the near future, it is expected that the unit magnetic reversal will be reduced to an equivalence to a single magnetized particle. Thus, in order to further reduce the unit magnetic reversal to ensure a specified S/N, a size of the magnetized particle itself must be reduced. Assuming that a volume of the magnetized particle is V, latent magnetic energy of the magnetized particle is expressed as KuV, where Ku is a magnetic anisotropic energy density of the particle. As a value V is decreased to reduce the noise, KuV is decreased, and this results in thermal disturbance in which thermal energy around the atmospheric temperature causes disturbance in recorded information.
According to the Analysis by Sharrock, a rate of the magnetic energy of the particle to its thermal energy, or KuV/kT (where Kt is a multiplication of k for a Boltzmann constant and T for an absolute temperature) should be a value of approximately 100, or otherwise, reliability in record lifetime would be deteriorated. As with Ku (2–3×106 erg/cc) of alloys containing CoCr radicals that have been used for medium magnetic film in the prior art, decrease in particle diameter for the purpose of noise reduction makes it more difficult to ensure durability to the thermal disturbance. Thus, recent approaches employ magnetic film materials such as CoPt and FePd that assume values of Ku of 107 erg/cc or over, but when Ku is simply raised to mutually establish both the reduction in the particle size and the durability to thermal disturbance, a further problem would be caused in relation with recording sensitivity. When Ku of the medium magnetic film is raised, record coercive force Hc0 of the medium is increased (where Hc0=Ku/Isb is defined, and Isb is a net magnetization of the medium magnetic film), and electric field required for saturation recording is increased in proportion to Hc0.
Recording electric field that is developed in the recording head and is applied to the medium depends upon conducting current flowing in recording coil, a material of recording magnetic pole, a shape of the magnetic pole, spacing, a type of the medium, a film thickness, and so forth, and allowing for a size reduction of a tip of the recording pole for the further density enhancement, developed electric field is finite
For instance, even when a single pole head developing the largest electric field is combined with a vertical medium having soft magnetic lining, the resultant recording magnetic field will have an upper limit as high as at most 10 kOe (Oe is an abbreviated symbol of oersted). On the other hand, in order to attain sufficient durability to thermal disturbance under the condition of the particle diameter as minute as 5 nm that should be required for a future model of the medium having further enhanced density and further reduced noise, it is necessary to use the magnetic film material having a density Ku of 107 erg/cc or above, and in such a case, the magnetic field required for recording in the medium around the atmospheric temperature is considerably greater than 10 kOe, and thus, the recording becomes impossible. Thus, there arises a problem that even the recording is impossible when only Ku in the medium is simply increased.
As has been described, as to the magnetic recording in the prior art poly-particle medium, noise reduction, assured durability to thermal disturbance, and assured recording sensitivity trade off for one another, and the limit of the recording density depends upon such trade-offs.